Management of patients with chronic disease consumes a significant proportion of the total health care expenditure in the United States. Many of these diseases are widely prevalent and have significant annual incidences as well. Heart failure prevalence alone is estimated at over 5.5 million patients in 2000 with incidence rates of over half a million additional patients annually, resulting in a total health care burden in excess of $20 billion. Heart failure, like many other chronic diseases such as asthma, COPD, chronic pain, and epilepsy, is event driven, where acute de-compensations result in hospitalization. In addition to causing considerable physical and emotional trauma to the patient and family, event driven hospitalizations consume a majority of the total health care expenditure allocated to the treatment of heart failure.
Hospitalization and treatment for an acute de-compensation typically occurs after the de-compensation event has happened. However, most heart failure patients exhibit prior non-traumatic symptoms, such as steady weight gain, in the weeks or days prior to the de compensation. If the caregiver is aware of these symptoms, it is possible to intervene before the event, at substantially less cost to the patient and the health care system. Intervention is usually in the form of a re-titration of the patient's drug cocktail, reinforcement of the patient's compliance with the prescribed drug regimen, or acute changes to the patient's diet and exercise. Such intervention is usually effective in preventing the de-compensation episode and thus avoiding hospitalization.
Patients with chronic heart disease can receive implantable cardiac devices such as pacemakers, implantable cardioverter defibrillators (ICDs), and heart failure cardiac resynchronization therapy (CRT) devices. Currently, the electrophysiologist that implants pacemakers and ICDs requires their patients to make clinic visits periodically, usually once every three or four months, in order to verify if their implanted device is working correctly and programmed optimally. Device follow-ups are usually performed by the nurse-staff assisted by the sales representative from the device manufacturers. Device follow-ups are labor intensive and typically require patients to make multiple clinic visits.
In an effort to limit the number of follow-ups necessary to monitor the device and the data that it acquires, an advanced patient management system may provide a communication infrastructure. This infrastructure allows the implantable medical device to communicate over long distances at virtually any time with a backend system that monitors the implantable device and the patient. Furthermore, this backend system allows monitoring of the patient on a more frequent basis than ordinary follow-up visits can practically allow. The back end system communicates with the implantable device through an external unit such as a repeater that the patient keeps in close proximity. Conventionally, the external unit communicates directly with the implantable device through inductive coupling which requires that the patient hold a wand over the location of the implant. The external unit then transfers information from the implantable device through a telephone line interface to the back end system.
The conventional approach to communicating with the implantable device has drawbacks in that the patient must actively participate in the communication process. The inductive coupling provides little range between the implant and the external unit, and therefore, the patient must be in close proximity during the communication process. However, radio frequency communications at higher frequencies than those used for inductive coupling are a viable option. Radio frequency communications provide much greater range between the external unit and the implant and allow the communication to occur automatically without patient intervention. However, providing automatic radio frequency communications abilities in the implant can lead to significant increases in cost, size, and power consumption.
The radio frequencies of transmission and reception by the implantable device are fairly unpredictable when using low-cost transceivers, such as surface acoustic wave (SAW) based devices, due to environmental effects and manufacturing tolerances. Therefore, the external unit cannot communicate with the implant at exactly the same frequencies each time. Furthermore, providing frequency synthesis in the implant to control the frequencies of transmission and reception is not a practical solution because frequency synthesis consumes more power than can be provided over a long-term basis, which is required by implants with lengthy life times. Therefore, radio frequency communications with the implant are troublesome.
Additionally, automatic and periodic radio frequency communications between the implant and external unit require that the implant and the external unit expect to communicate at the same time. The communication must be periodic to reduce the power consumption required by the communications devices within the implant. However, environmental effects and manufacturing tolerances cause time drift between the clock of the implant and the clock of the external unit. The uncertainty as to the proper time to communicate can cause the implant to power up in anticipation for communication at a different time than when the external unit attempts to communicate. Therefore, opportunities to communicate may be completely lost because the devices are not properly synchronized.